We propose to study the nature of human chromosomal diseases which have predisposition to cancer. Emphasis for now will be placed with three of these diseases, namely: Ataxia telangiectasia, Fanconi's anemia and Xeroderma pigmentosum. Our hypothesis is that deficiency in DNA repair is an underlying cause; each disease may have a different but related defect. The defects are to be examined biochemically and via somatic genetic techniques. Fundamental cellular activities related to DNA repair are also being studied.